As with concern about the depletion of oil resources, there is an urgent need for the development of a plant-derived resin from petroleum. With respect to the plant-derived resin, attention has been paid to a lignin derivative obtained from pulp by-product lignin and a lignin derivative obtained from a subcritical water treatment of biomass, a so-called organosolv type lignin derivative obtained by digestion of biomass using an organic solvent, and lignin by-produced from bio-ethanol production processes. Hexamethylenetetramine, known as a curing agent for a novolac-type phenolic resin, is added to the obtained lignin derivative which is then cured by heating, whereby it is possible to obtain a lignin resin cured product (see PTL 1).
However, a cured product obtained by adding hexamethylenetetramine to a single lignin derivative, followed by curing, has suffered from a problem of a lower bending strength as compared to a molded article of a novolac-type phenolic resin.